1. Field of the Invention
The present invention generally relates to an ultrasonic sensor having a piezoelectric element.
2. Description of the Related Art
An ultrasonic sensor is configured to generate an ultrasonic wave and receive a part or the entire generated ultrasonic wave reflected back to the ultrasonic sensor. The ultrasonic sensor is used in, for example, an obstacle detecting system that detects an object by intermittently generating an ultrasonic wave in a particular direction(s) and receiving a part or the entire generated ultrasonic wave reflected from an obstacle situated in this direction. The ultrasonic sensor may also be used as a range finding system configured to measure the distance from a target object. The range finding system, which uses the ultrasonic wave sensor to transmit an ultrasonic wave and receive a reflected ultrasonic wave in response to the transmitted ultrasonic wave, measures the difference in time between the ultrasonic wave being transmitted and the reflected ultrasonic wave being received and calculates the distance from a target object according to the measured time difference.
FIG. 7 shows an exemplary configuration of a conventional ultrasonic sensor 10.
The ultrasonic sensor 10 mainly includes a sensor case 11, a piezoelectric element 12, a pair of terminals 13, and an acoustic material 15. The sensor case 11 is made of aluminum and has a closed-bottom cylindrical shape. A bottom part 11a of the sensor case 11 functions as a vibration plate.
The piezoelectric element 12 has an electrode formed on each its upper surface and its lower surface (hereinafter referred to as “upper electrode” and “lower electrode”). In a case where voltage is applied to this pair of electrodes, the piezoelectric element 12 generates mechanical energy (stress causing vibration or strain). In a case where mechanical energy (vibration) is applied to the piezoelectric element 12, an electrical charge is generated between the pair of electrodes. The piezoelectric element 12 is positioned at the bottom part 11a functioning as a vibration plate of the sensor case 11.
Furthermore, the upper electrode of the piezoelectric element 12 is electrically connected to one end of the terminal 13 by a wire 14. The lower electrode of the piezoelectric element 12 is electrically connected to the bottom part 11a of the sensor case 11 made of aluminum. The sensor case 11 and the other end of the terminal 13 are electrically connected by using a wiring pattern 16.
Thereby, the piezoelectric element 12 is electrically connected to the pair of terminals 13. It is to be noted that the acoustic material 15 is placed inside the sensor case 11 for preventing reflection of ultrasonic waves (See Japanese Laid-Open Patent Application Nos. 9-284896, 11-266498, 2000-32594, and 2002-209294).
The conventional ultrasonic sensor typically uses aluminum for its sensor case for creating vibration by mechanical resonance caused by the piezoelectric element. However, due to the characteristics of aluminum material, it is difficult to perform soldering on the aluminum-made sensor case and unsuitable to coat the aluminum sensor with a conductive adhesive agent because the conductive adhesive agent requires a secondary process such as curing by heat.
Therefore, in a case of connecting the aluminum case to a wire or a terminal board, there is proposed a method of providing an engagement part on the sensor case and soldering the wire or the terminal board to the aluminum case via the engagement part by using a connecting pin or a wiring pattern. However, such a method requires a step of forming the engagement part, generates costs for manufacturing the connecting pin, and is difficult to assemble.